A liquid crystal display indispensably requires polarizing elements disposed on both sides of a liquid crystal cell because of an image forming method adopted therein and generally polarizing plates, which comprises a transparent protective film on one side or both sides of a polarizer, are adhered. Besides, on a liquid crystal panel, various kinds of optical elements have been used in addition to a polarizing plate in order to improve a display quality of a display. For example, there have been used a retardation plate for coloration prevention, a viewing angle increasing film for improving a viewing angle of a liquid crystal display and a brightness enhancement film for raising a contrast of a display. The films each are collectively referred to an optical film.
A pressure-sensitive adhesive is usually employed in adhering an optical film described above to a liquid crystal cell. An optical film and a liquid crystal cell or optical films are usually adhered to each other using a pressure-sensitive adhesive therebetween in order to reduce a light loss. In such cases, a pressure-sensitive adhesive optical film in which a pressure-sensitive adhesive is provided in advance on one surface of an optical film as a pressure-sensitive adhesive layer is generally used because of a merit such as that no necessity arises for a drying step of fix the optical film. A pressure-sensitive adhesive optical film is also used in which a pressure-sensitive adhesive layer is provided on an optical film with an anchor layer interposed therebetween for improving the anchoring properties of the pressure-sensitive adhesive layer to the optical film (see Patent Literature 1 below).
As image display applications such as televisions, monitors, car navigation systems, and cellular phones have expanded, the pressure-sensitive adhesive optical film has been required to have high durability under various environmental conditions, such as no change in optical properties even in cases where it is allowed to stand for a long time in a high-temperature, high-humidity environment (no decomposition or degradation of transparent protective films or no change in polarization properties in the case of polarizing plates) and no occurrence of appearance defects such as foaming or peeling of the pressure-sensitive adhesive layer. In particular, due to an increase in the size, brightness or definition of liquid crystal displays, a demand for appearance improvement becomes stronger than that for conventional pressure-sensitive adhesive optical films.
A method of reducing the acrylic acid content of a pressure-sensitive adhesive (see Patent Literature 2 below) and a method of adding a tertiary amine to a pressure-sensitive adhesive (see Patent Literature 3 below) are known for the purpose of preventing decomposition or degradation of the protective film. In these methods, however, alterations in the polarization properties of polarizing plates or foaming or peeling of pressure-sensitive adhesive layers cannot be prevented.
The pressure-sensitive adhesive layer or the anchor layer is conventionally formed using a solvent-dispersible material but is often formed using a water-dispersible material recently, because of environmental problems.
Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 10-20118
Patent Literature 2: JP-A No. 59-111114
Patent Literature 3: JP-A No. 04-254803